Some physicists attempting
to unify gravity with the other fundamental
forces have come to a startling prediction: every
fundamental matter particle should have a massive
"shadow" force carrier particle, and every force carrier should
have a massive "shadow" matter particle. This relationship
between matter particles and force carriers is called
supersymmetry.
For example, for every type of quark there may be a type of particle called a
"squark."

No supersymmetric particle has yet been found, but experiments
are underway at CERN and
Fermilab to detect supersymmetric partner particles.